This exact problem happened to me. Dropped it. Volume up button does not click. SanDisk refused to accept my warrenty claim because of “physical damage”. I hope they treat you better. My old Samsung mp3 player is over 4 yrs old, been dropped many times, and is still working. Sadly though my Samsung only has 0.5 GB.
Since SanDisk refused my warrenty claim, I took the repair into my own hands. I opened the unit (voiding the now defunct warrenty) and found the problem. The volume up switch became detached from the circuit board. I soldered it back on, put the unit together, and now the volume up button works again.
From an engineering perspective, the volume up and down buttons are poorly designed. Every time a user depresses these buttons, it puts a small amount of stress on the soldered connection to the circuit board. Over time, these may fail. The power button has the same design.
In summary, I hope your SanDisk warrenty takes care of you. Otherwise, I hope you or someone you know is handy with a soldering iron on circuit boards. Opening your Clip+ will void your warrenty.
ElPinguino (Sansa Clip+ 8GB)